This invention relates to an improved method of measuring the vacuum pressure in a sealed glass, or other electrically non-conductive vessel.
Perishable products are frequently packaged in evacuated glass vessels.
In order to ensure the integrity of the product supplied to the customer, it is usually necessary identify and reject vessels that are no longer properly evacuated due to leakage.
A known method of confirming the existence of a suitable vacuum within the vessel, is to use a high voltage, high frequency source, eg. a Tesla coil, to create an alternating electric field that ionizes the residual gas molecules within the vessel, if it is sufficiently evacuated.
The ionization can be detected, by the light emission from the discharge within the vessel, by registering the increase in current delivered by the high voltage source, or by registering a drop in voltage of the said source.
U.S. Pat. No. 4,546.319, proposes a method of deducing a quantitative value for the vacuum pressure within the vessel by measuring the value of electrode current and comparing it with a reference vial of a known pressure.
In practice, the above method is prone to considerable inaccuracy due to the fact that only a portion of the electrode current measured by the device is accounted for by the electrical discharge within the vial. Additional current due to stray capacitance and earth leakage can considerably distort the results. A further difficulty involved with quantitative measurement by this method is that the electrode current is non-linear with respect to vial pressure.
Patent DE 41 26 275 A1 discloses a non-invasive method of measuring internal pressure in which the electrode voltage is increased progressively, and the voltage at which ionization initially occurs is utilised as the parameter indicative of vial pressure.
In practice, the indeterminate time delay between the application of a suitably high voltage and the start of ionization introduces measurement errorsxe2x80x94unless the voltage ramp rate is sufficiently slow that the delay variation becomes small compared to the overall ramp time.
Although DE 41 26 275 A1 discloses means to minimize the ionization delay, the delay is not eliminated and still introduces progressively larger errors as the voltage ramp rate is increased, such as would be required in the case of a high speed automatic testing process.
The current invention aims to provide a fast and accurate method of measuring vacuum pressure in sealed vials.
According to the present invention, there is provided, a method of measuring the vacuum pressure within a sealed vial or vessel comprising the following steps:
a) Placing the vial between a pair of electrodes.
b) Ionizing the gas contained within the vial by applying a high frequency, high voltage potenial across the electrodes.
c) Progressively decreasing the voltage once ionization has occurred.
d) Detecting the termination of ionization.
e) Deriving the gas pressure within the vial as a function of the electrode voltage at the point at which ionization terminates.
Detecting the termination of ionization may be by registering either the sudden drop in electrode current, or the termination of light emission from the vial. The point at which there is a sudden drop in electrode current or the light emission terminates, coincides with the termination of ionization.
In the method, the voltage at which ionization ceases is then measured. The vial pressure is proportional to the electrode voltage at which ionization ceasesxe2x80x94herein referred to as the xe2x80x98drop-out voltagexe2x80x99
The method produces a more accurate and consistent result than measuring the current at a given voltage as in U.S. Pat. No. 4,546,319 since the method is not affected by variations in leakage current or stray capacitance.
Unlike the method described in DE 41 26 275 A1, the present invention is not subject to errors due to time delay or hysteresis at the start of ionization. The ionization ceases instantaneously and repeatably once the voltage falls below that required to maintain a discharge. This enables a fast voltage ramp rate and thus rapid testing without compromising accuracy.
In the embodiment of the invention herein described, the drop-out voltage is measured by detecting the sudden drop in electrode current as ionization ceases.
Alternatively, the end of ionization may be detected optically by the termination of light emission from the vial.